1. Field of the Invention
The present invention relates to an ink tank configured to store recording liquid, such as ink, and a recording apparatus including the ink tank. The present invention can be applied to various recording apparatuses having ink tanks. For example, the present invention can be applied to a typical printer, a copy machine, a facsimile having a communication system, a word processor having a printing unit, and an industrial recording apparatus integrated with various other processing apparatuses.
2. Description of the Related Art
As a recording apparatus that uses ink stored in an ink tank, for example, an inkjet recording apparatus using an inkjet recording head for discharging ink is known. As such an inkjet recording apparatus, a serial scanning type inkjet recording apparatus that includes a carriage holding an inkjet recording head and an ink tank and moves in the primary scanning direction is known.
Such a serial scanning type inkjet recording apparatus includes a carriage that supports an inkjet recording head and an ink tank for supplying ink to the recording head. When recording is carried out, the carriage is moved relative to a recording medium, and ink droplets are discharged from minute outlets provided on the recording head. The ink droplets land on a recording medium to form a predetermined image.
For the inkjet recording head, dye ink including dye as a colorant has been typically used. However, the performance of dye ink is not sufficient when used for recording images on a recording medium to be displayed outdoors because, when displaying an image outdoors, it is important for the ink to have good light resistance and good whether resistance. To replace dye ink in such a case, pigment ink including pigment as a colorant has been provided. However, when pigment ink is used, the pigment particles are deposited at the bottom of the ink tank because pigment ink forms a dispersal system, not a soluble system.
For example, if the inkjet tank is mounted on the inkjet recording apparatus and left untouched for a long period of time, the pigment particles in the ink are gradually deposited in the ink tank. Therefore, a concentration gradient of the pigment particles is generated in a direction from the bottom to the upper area of the ink tank. As a result, the concentration of the pigment particles in the ink at the lower area of the ink tank is increased. Consequently, a layer of highly concentrated pigment particles is formed at the bottom portion of the ink, whereas a layer of less concentrated pigment particles is formed at the upper portion of the ink.
If the ink inside the ink tank is guided outside through the bottom of the ink tank and supplied to a recording head, first, the ink in the highly concentrated pigment layer is supplied, causing an excessively dark image to be formed. In other words, the image formed during early stages of the life of the ink tank and the image formed during a later stage of life of the ink tank may have differences in darkness noticeable by the human eyes. Such differences are especially noticeable when a color image is formed with dark and light colors.
Japanese Patent Laid-Open No. 2004-216761 describes a structure for stirring ink inside an ink tank by moving a stirrer by inertial force generated by the reciprocal movement of a carriage.
More specifically, Japanese Patent Laid-Open No. 2004-216761 describes an ink tank that accommodates a freely shakable stirrer. The center of shaking of the stirrer is set substantially in the center of the ink tank in the direction of the carriage's movement. The stirrer shakes by the same amount in opposite directions when the carriage is reciprocated. Japanese Patent Laid-Open No. 2004-216761 also describes a structure in which a freely movable spherical weight is provided at the bottom of the ink tank and a structure in which a fixed stirring wall that disturbs the ink flow in the ink tank.
However, the ink tanks described in Japanese Patent Laid-Open No. 2004-216761 have the following problems.
When a freely shakable stirrer is provided inside the ink tank, the stirrer shakes by the same amount in opposite directions, and the shaking of the stirrer is centered on substantially the center of the ink tank. Thus, in order to increase the stirring performance by increasing the range of shaking of stirrer, the width of the ink tank has to be increased in the direction of the carriage's movement. However, since a plurality of ink tanks is often mounted on the carriage along the direction of the carriage's movement, the width of each ink tank is limited to a relatively small width. For this reason, the shaking range of the stirrer cannot be increased, and, thus, the ink flow generated by the shaking of the stirrer cannot be increased. To sufficiently stir the ink, the number of times the carriage is reciprocated and the amount of stirring time have to be increased.
The ink tank having a freely movable spherical weight and the ink tank having a fixed stirring wall have simple structures. However, for the former ink tank, it is difficult to sufficiently stir the entire volume of ink, including the ink close to the upper surface of the ink tank, by merely moving the weight provided at the bottom of the ink tank. For the latter ink tank, a fixed stirring wall is provided to disturb the ink flow on the presumption that an ink flow is generated. In other words, the fixed stirring wall does not actively generate an ink flow. Furthermore, both of the ink tanks do not effectively operate unless structures allowing air to be taken in to the ink storage unit in accordance with the decrease in the volume of ink are provided. In other words, a space is created inside the ink storage unit as air is taken in, and this space is used to move the ink while the carriage is being reciprocated.
However, some ink tanks have structures that do not allow air to be taken in. The only amount of air the ink storage unit of such an ink tank accommodates is some bobbles of air that enter during production and shipment. Therefore, there is not enough space (air) for the ink to move inside the ink tank. Accordingly, it is difficult to generate a strong flow of ink by reciprocating the carriage and/or moving a spherical weight.
These problems in the ink tank described in Japanese Patent Laid-Open No. 2004-216761 are also apparent from the structure of a typical ink tank and recording apparatus described below.
In general, an ink tank held by a carriage (which is known as an “on-carriage ink tank”) has a predetermined width and a predetermined length to improve the detachment and attachment of the ink tank. More specifically, the width of the ink tank in the direction of the carriage's movement (i.e., primary scanning direction) is relatively small, whereas the length of the ink tank in the conveying direction of the recording medium (i.e., secondary scanning direction orthogonal to the primary scanning direction) is relatively great. Consequently, the stirrer cannot be moved a great distance in the primary scanning direction, which is the displacement direction of the stirrer. As a result, the stirrer is moved only a small distance, preventing a strong flow of ink from being generated in the ink tank. Accordingly, the ink-stirring efficiency is reduced, and a great amount of time will be required to stir the entire volume of ink in the ink tank. For example, if pigment particles in the ink are deposited at the bottom of the ink tank because recording had not been carried out by the recording apparatus for a long period of time while the ink tank was attached to the carriage, the carriage will have to be reciprocated for a long period of time before carrying out recording again. As a result, the warm-up time required for enabling a recording operation will become long. In particular, when the pigment particles in the pigment ink are great and/or when the relative density of the pigment particles is great, the pigment particles are deposited quickly. Therefore, a concentration distribution causing negative effects on the image formed by the recording apparatus may be generated inside the ink tank by merely leaving the ink tank untouched for several days. To prevent this, the ink in the ink tank must be stirred every several days. However, while the ink is being stirred, image formation cannot be carried out.